This invention relates in general to valves for controlling fluid flow in a hydraulic, pneumatic, or heating, ventilating, air conditioning, and refrigeration (HVAC/R) system. In particular, this invention relates to an improved hybrid spool valve assembly that reduces leakage in such a hydraulic, pneumatic, or HVAC/R system.
Valves are widely used for controlling the flow of fluid from a source of pressurized fluid to a load device or from a load device to a pressure vent. Frequently, a pump or other device is provided as the source of pressured fluid. The flow of the fluid may be selectively controlled by a valve to control the operation of the load device. One type of valve used to selectively control the flow of the fluid is a hybrid spool valve assembly that may include a macro-sized main spool valve that is driven by a pilot microvalve.
One known macro-sized main spool valve that is driven by a pilot microvalve is an expansion valve, such as a Modular Silicon Expansion Valve (MSEV). MSEVs are electronically controlled, normally closed, and single flow directional valves. MSEVs may be used for refrigerant mass flow control in conventional HVAC/R systems and other hydraulic or pneumatic systems.
The MSEV is a two-stage proportional control valve. The first stage is a microvalve that acts as a pilot valve to control a second stage spool valve. When the microvalve receives a Pulse Width Modulation (PWM) signal, the microvalve modulates fluid flow to change the pressure differential across the second stage spool valve. The spool valve will move to balance the pressure differential, effectively changing the size of the orifice opening of the MSEV to control the desired amount of refrigerant flow.
A microvalve is an example of a micro-electro-mechanical system. Generally speaking, a micro-electro-mechanical system is a system that not only includes both electrical and mechanical components, but is additionally physically small, typically including features having sizes in the range of ten micrometers or smaller. The term “micro-machining” is commonly understood to relate to the production of three-dimensional structures and moving parts of such micro-electro-mechanical system devices. In the past, micro-electro-mechanical systems used modified integrated circuit (e.g., computer chip) fabrication techniques (such as chemical etching) and materials (such as silicon semiconductor material), which were micro-machined to provide these very small electrical and mechanical components. More recently, however, other micro-machining techniques and materials have become available.
As used herein, the term “micro-machined device” means a device including features having sizes in the micrometer range or smaller and, thus, is at least partially formed by micro-machining. As also used herein, the term “microvalve” means a valve including features having sizes in the micrometer range or smaller and, thus, is also at least partially formed by micro-machining. Lastly, as used herein, the term “microvalve device” means a micro-machined device that includes not only a microvalve, but further includes additional components. It should be noted that if components other than a microvalve are included in the microvalve device, these other components may be either micro-machined components or standard-sized (i.e., larger) components. Similarly, a micro-machined device may include both micro-machined components and standard-sized components.
A variety of microvalve structures are known in the art for controlling the flow of fluid through a fluid circuit. One well known microvalve structure includes a displaceable member that is supported within a closed internal cavity provided in a valve body for pivoting, axial, or other movement between a closed position and an open position. When disposed in the closed position, the displaceable member substantially blocks a first fluid port that is otherwise in fluid communication with a second fluid port, thereby preventing fluid from flowing between the first and second fluid ports. When disposed in the open position, the displaceable member does not substantially block the first fluid port from fluid communication with the second fluid port, thereby permitting fluid to flow between the first and second fluid ports.
U.S. Pat. Nos. 6,523,560; 6,540,203; and 6,845,962, the disclosures of which are incorporated herein by reference, describe microvalves made of multiple layers of material. The multiple layers are micromachined and bonded together to form a microvalve body and the various microvalve components contained therein, including an intermediate mechanical layer containing the movable parts of the microvalve. U.S. Pat. No. 7,156,365, the disclosure of which is also incorporated herein by reference, describes a method of controlling the actuator of a microvalve. In the disclosed method, a controller supplies an initial voltage to the actuator which is effective to actuate the microvalve. Then, the controller provides a pulsed voltage to the actuator which is effective to continue the actuation of the microvalve.
It is known in the art that a spool valve driven by a pilot microvalve may experience at least some fluid leakage through the spool valve outlet even when the spool valve is closed or in a power off condition. Manufacturers of some fluid systems, such as refrigeration systems, require a lower level of leakage in the spool valve than may be provided by a conventional spool valve to prevent fluid from flowing back to the compressor. Often, refrigeration system manufacturers will add an additional valve, such as a solenoid valve to the refrigeration system to positively shut-off the flow of any fluid that may leak from a conventional spool valve. It would be desirable however, to provide an improved structure for a hybrid spool valve assembly that reduces or eliminates undesirable leakage of the fluid flowing therethrough without the need for an additional valve in the fluid system.